1. Field of the Invention
The invention relates to an optical system for an image pickup electronic endoscope in which there is a solid-state image sensor disposed within a stiff distal end of the endoscope.
2. Description of the Prior Art
As is well recognized, a solid-state video camera may be formed by charge coupled devices (CDD) which are arranged in the form of a dual in-line IC package. The package has an imaging plane or input surface defined in a part of the top surface thereof, and is internally assembled with peripheral circuits. Accordingly, the overall size of the video camera will be as large as several times the dimension of the imaging plane. When it is desired to construct an endoscope utilizing such a solid-state video camera for taking a picture, a difficulty is experienced in the arrangement of the camera. Specifically, FIG. 2A shows the distal end 1 of an endoscope in which an objective lens assembly 2 is disposed. If a solid-state video camera 3 is disposed rearwardly of the lens assembly 2 in an orientation so that it is perpendicular to the length or the axial direction of the endoscope, the size of the solid-state video camera 3 requires that the diameter of the distal end 1 of the endoscope be increased, as illustrated in FIG. 2B, which is undesirable. Accordingly, to overcome such difficulty, an arrangement is disclosed in Japanese Laid-Open Patent Application No. 46,922/1983 in which a reflecting member 4 such as a prism as shown in FIG. 3A is disposed rearwardly of the objective lens assembly 2 within the distal end 1 of the endoscope so that the optical axis extending from the lens assembly 2 may be bent in a direction perpendicular to the length or the axial direction of the endoscope, thus permitting light for observation to impinge upon a solid-state video camera 3' which is now disposed so as to extend parallel to the axis of the endoscope. However, it will be appreciated from FIG. 3B that in this instance, because the solid-state video camera 3' is offset from the optical axis of the lens assembly 2, the camera 3' cannot be disposed in proximity to the center axis of the distal end 1, and hence stiff distal end 1 must still be of relatively large diameter so that the patient will be subjected to discomfort.